In the past, electric generators or motors having a rotor in which a field coil is disposed, a stator in which an armature coil is disposed so as to be opposed to the field coil and a rotating shaft connected and fixed to the rotor have been used. In case of this type of synchronous machine, the rotating field coil has been supplied with electric power by a slip ring connected to one end of the rotating shaft (for example, see Patent Documents 1 and 2).
Patent Document 1: Japanese Patent Application, First Publication No. 54-116610
Patent Document 2: Japanese Patent Application, First Publication No. 6-86517